One of the features that I absolutely love about GIMP is the ability to dynamically change the threshold while making selections with the Fuzzy Select and Select by Color tools, by dragging the mouse while making the selection. Since 2.8.0 however these tools have become so slow that it has become unpractical to try to change the threshold, even on small images. The Fuzzy Select seems OK in the latest version, but Select by Color is still very slow.
Try opening the attached image, and hold down the Select by Color tool anywhere in the gradient, and then start dragging the mouse. In GIMP 2.6 the selection would grow and shrink instantly. Now it takes at least a second between updates.